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INDI Library v1.9.5 Released (08 Mar 2022)

Bi-monthly INDI Library released with new drivers and bug fixes.

Drift Plot of Guide Module

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I wonder what is shown in the drift plot of the Ekos guide module. The deviation from the lock position of the calibration or the deviation from last guiding position measurement.
If it is the deviation from last position measurement then the centre of all plot points would define something like a mean drift rate.
KStars 3.5 + astroberry remote
EQMOD, EQ6-R , ASI2600+ ASI178+EAF+ASI120
Last edit: 1 year 3 months ago by Arnold Graf.
1 year 3 months ago #66116

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What is plotted is the difference between the measured position and the "lock position" or "target" or "guide star position", or whatever you want to call it. This difference tuple (also called "drift") is what's used when calculating the correction pulses to be sent to the RA and DEC motors--units are arc-seconds, and axes point in the RA and DEC directions (as determined by calibration).

This lock position is set at the start of guiding, and changes when dithering, or when the guiding restarts due to a slew, meridian-flip, or lost guide-star. So, the ideal value is 0,0 meaning, the measured position is exactly where we want it to be.

The same values are plotted on the "Drift Plot" and "Drift Graph" of the Guider tab, as well as the RA and DEC graphs in the Analyze tab.

BTW, calibration is something else--it does not determine the lock position. Often, though, the lock position is set right after calibration.
AP1100 & Orion Atlas Pro, WO/ZS105 w/Moonlight V2 focus, GSO RC10 w/RSF focus
ZWO ASI1600, Astronomik Filters, ST80, QHY 5L-IIm.
KStars/Ekos/Indi on NUC10 & RPi4 w/SSD -- Ubuntu
Projects: Greedy Scheduler, Terrain, Polar Align, Analyze, Linear Focuser, SEP MultiStar & GPG Guide, FITS autostretch.
1 year 3 months ago #66127

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If you are interested in systematic drifts of the pointing (from PAE or wrong mount speed) you can look at the correction pulses sent to the mount, best done in the analyze tab. If there is some drift they will have more corrections in one direction than in the other, and the mean of the pulse timings will tell you that drift. E.g., here:


I have a 5' PAE, so I get drift in DEC, and the pulses (blue) are almost all in positive direction....
openSUSE Tumbleweed KStars git INDI git
GPDX+EQMOD, CEM60EC, ASI2600/1600/290mini+EFW+EAF
1 year 3 months ago #66139
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Many thanks for the comprehensive explanations and hints. My question came up when I detected bad pixel traces in a stacked picture even though the Drift Plot looked ok.
As I use OAG guiding it is hard to imagine that there is a major relative movement between the main camera and the guide camera.
Any idea about the root cause of such bad pixel traces?
KStars 3.5 + astroberry remote
EQMOD, EQ6-R , ASI2600+ ASI178+EAF+ASI120
1 year 3 months ago #66160
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If you use an OAG it is most likely polar alignment error. That will cause image rotation, and while the angular speed is always the same, the absolute amount will depend on the distance from the lock point, i.e., the guide star. Now with an OAG the guide star is (far) outside the FOV, so the effect of such a rotation is much easier to see than when the guide star were in the center of the (main) camera frame....

As the amount of image rotation depends on declination, it is much more visible when imaging closer to the pole. If you have several target series that show this, it might confirm the guess that it is PAE...
openSUSE Tumbleweed KStars git INDI git
GPDX+EQMOD, CEM60EC, ASI2600/1600/290mini+EFW+EAF
The following user(s) said Thank You: Arnold Graf
1 year 3 months ago #66189

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I'm sure you are right. When I made these series I had a significant PAE (26') because they were part of test cases to challenge a bit the guiding capabilities of Ekos. But I was not aware, that the resulting image rotation could not be fully compensated by OAG guiding.

But next time I will be ware.
Many thanks again!
KStars 3.5 + astroberry remote
EQMOD, EQ6-R , ASI2600+ ASI178+EAF+ASI120
1 year 3 months ago #66190

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Replied by Paul on topic Drift Plot of Guide Module

Just wondering if you could use the drift plot to analyze and adjust your polar alignment. If you see a consistent drift in a certain direction, it's quite likely there is an error in polar alignment, no ?
9 months 4 days ago #74453

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Agreed, though it's tricky because if you analyze part way through a worm gear cycle, then periodic error can masquerade as polar align offset.

The guider does save a guide log in a sister directory to the main logs, and that log is compatible with the phdlogviewer program that the phd2 folks provide. They do these sorts of analyses in that program.

Hy
AP1100 & Orion Atlas Pro, WO/ZS105 w/Moonlight V2 focus, GSO RC10 w/RSF focus
ZWO ASI1600, Astronomik Filters, ST80, QHY 5L-IIm.
KStars/Ekos/Indi on NUC10 & RPi4 w/SSD -- Ubuntu
Projects: Greedy Scheduler, Terrain, Polar Align, Analyze, Linear Focuser, SEP MultiStar & GPG Guide, FITS autostretch.
9 months 4 days ago #74454

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Shit. Just wrote a longish reply, then the server wasn't responding and 'go back' in the browser doesn't restore the typed text :(

Nevertheless, I'll try again.
PAE is (AFAIK) derived from the DEC drift only. If - as Paul suggests - the guided info is used, there is no influence of PE on this, as it's corrected out. One has however to sum the single DEC corrections and delta t, and ignore the dithers properly. I had a quick try on that using some guide log from a few days ago:

 

The drifts are quite small here, as my mount sits on a permanent pier and is quite well polar aligned.  And I have no idea what causes the reversal of drift direction, TBH....  (*) But from that data/graph one can extract the DEC drift in "/min and with the proper formula (that I don't have at hand ATM) compute the PAE.

Is having that available live in the guide tab helpful? Maybe. PHD2 doesn't have it, and usually one would rather use the separate polar drift align and actually do the PA. However, it might be interesting for people that set up their equipment temporarily and on softer ground. There, PAE could build up during the night, like from the tripod sinking in, or from walking around close to the telescope...

(*) Edit:  Just thought about that:  It probably is the discrimination between AZ and ALT error:  The early part is 'close to horizon' and thus measures ALT error, whereas the later part is close to meridian - end of guiding was actually due to meridian flip - where drift align is sensitive to AZ error....
openSUSE Tumbleweed KStars git INDI git
GPDX+EQMOD, CEM60EC, ASI2600/1600/290mini+EFW+EAF
Last edit: 9 months 3 days ago by Peter Sütterlin.
9 months 3 days ago #74472
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